As a source, I think it would make sense to have a negative smerit balance until the next cycle.
How about the threshold of borrowable sMerits?
But even there is threshold for it, I don't think it is a good approach.
First, I could be wrong, but from my perspective and my understandings on how merit system works and the active operations of merit sources, sometimes good posters have to wait for days, weeks, or longer to see sMerits rewarded to their good posts. If a post is a good one, it might be missed today, next weeks, but at the end, someone will see it and merit it.
I know, merit sources don't have time to scan around the forum, to find good posts and merit them immediately. If they don't merit good posts instantly, they might forget because lack of bookmarking. It will cause some kind of unfairness for good posters with some of their good posts (not all, definitely).
Another disavantage of the suggestion is the ability to self-control meriting behaviours from merit sources who want to borrow sMerits before their smerit pool refilled. Naturally, when more sources are available, it will somewhat result in more easily meriting activities. That is what we have witnessed so far. Before the increases of 30-days smerit pool to all merit sources, some sources usually give away 1 sMerit to 1 good post, but now they easily to give away 2 or 4 sMerits to 1 good post. I don't say it is wrong but if the smerit pool increases 4 times, they will send 4-fold amount of sMerits to the same post, or different post with same level of post quality. In the long run, it will reduce the level of post-quality of merited posts, IMO.
I meant, one year ago, a good poster has to make more than 100 good posts and need at least 100 of those good ones to be merited with at least 1 merit per post to earn 100 merits. Now, they have to make around 50 good posts or 60 ones, and will be rewarded 100 merits, with 1 or 2 merits per post.
The last one is good postes have to practice for their patience. They need to be as patient as possible, and focussing on their post quality, and contributions, patience will help them in the long journey.